The New York Stock Exchange and the Nasdaq Stock Market Inc. are racing to become the world’s first transatlantic stock trading center.

It’ll be an expensive, time-consuming proposition and a fair amount of risk for stock exchanges on both sides of the ocean.

And for the average investor, it could mean very little for quite awhile.

NYSE Group Inc.’s $10.2 billion cash-and-stock offer Monday for European exchange operator Euronext NV would create a $21 billion company called NYSE Euronext with trading in stock, corporate bond, futures, options, derivatives and commodities on two continents.  

Euronext runs the Paris, Brussels, Amsterdam and Lisbon exchanges. 

“NYSE Euronext will be the world’s most liquid and truly global financial marketplace, offering unparalleled benefits for investors and issuers in the United States, Europe and across the globe,” said NYSE Group Chief Executive John Thain, who would be CEO of the combined company if approved.

Euronext’s board was receptive to the bid, saying Monday it was the most attractive offer on the table, implicitly better than a competing bid from Deutsche Bourse AG made Friday. Euronext shareholders will meet Tuesday in Amsterdam to consider the offers.

Meanwhile, the Nasdaq, rebuffed by the London Stock Exchange in a $4.5 billion bid March 30, has been buying up London exchange shares and now owns 25.1 percent of the London market. While it waits the six months required under British law to make another bid, the stake gives the Nasdaq some veto power over major changes at the London exchange — though not enough to stop a competing acquisition bid.

Such mergers have the potential to create financial powerhouses — the NYSE could become a global market for a variety of investments, and the Nasdaq is poised to become a worldwide, 24/7 trading platform for nearly any stock.   

The exchanges could expand their reach and lower their costs, and if that saving is passed on to the rest of the investing public, that might be the only impact should either acquisition, or both, be completed.

There are certainly benefits to be had. Nasdaq and the New York exchange could end up trading American and European shares on both sides of the Atlantic, creating an extended trading day and improved revenues.

The Nasdaq could corner the British market in trading U.S. equities, while the NYSE could use Euronext’s technologies to more quickly upgrade its aging trading systems.

Yet there’s some danger of backlash from major Wall Street financial companies should the NYSE and Nasdaq succeed.

Analysts say that if the NYSE becomes a critical trading center for stocks and myriad other investments, there’s a concern that the exchange could raise its transaction fees, since it has less competition from other exchanges. Even a tenth of a penny adds up for a Wall Street firm making hundreds of thousands of transactions every day.

The Nasdaq’s efforts, which are more about becoming the world’s premier stock trading platform than offering a diverse line of investments, could raise similar issues.

At best, it would take months, if not years, for the companies to combine and standardize their trading technologies. It would take longer still for either the NYSE or Nasdaq to gain regulatory approvals in both Europe and the United States to allow cross-continental trading.

“There are going to be some improvements, certainly,” Easthope said. “But the average person isn’t going to see any great changes for quite a while.”

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On the Net:

NYSE Group Inc.: www.nyse.com

Euronext NV: www.euronext.com